We wish to understand the response of mammalian cells to treatment with mutagenic and carcinogenic chemicals. What is the mechanism of the mutation process in mammalian cells, how does the genome react with chemical agents, what is the response of the cell to the reaction products, what genetic sytems control the response of the cell to the mutagens? Since the initial events in carcinogenesis and mutagenesis may result from abnormal DNA replication in the presence of lesions not removed by excision repair, we wish to understand the interactions between excision repair and this abnormal replication in human lymphoblastoid cells and to determine the reason for the "error-prone" nature of certain parts of the recovery process. We therefore propose to: 1) Compare replication intermediates observable by electron microscopy in lymphoblastoid lines in which DNA replication is inhibited, with normal cells and with lines derived from individuals with xeroderma pigmentosum. We also intend to compare the replication intermediates from fibroblasts derived from normal, excision defective xeroderma, and "variant" xeroderma patients deficient in post replication repair. 2) Utilize purified and crude DNA polymerase preparations from normal cells to reconstruct an "error prone" process in vitro, in order to determine whether a random base addition mechanism could account for the biological phenomena in post replication repair. 3) Isolate the newly synthesized DNA formed in vivo by cells damaged with an alkylating agent in order to ascertain whether such DNA contains an unusually high number of mismatched base pairs; and 4) Determine the relative extent of excision repair in cells from different organisms and tissues by the BND cellulose method to study the interaction between the different repair mechanisms.